1. Field of the Invention
The present invention relates to computer network construction and operation, and in particular to network connection admission control (NCAC) for end-to-end quality of service (QoS) in packet-switched networks.
2. Description of the Prior Art
Business pressures to reduce the capital and operating expenses of building and operating separate network infrastructures have resulted in a general migration by service providers toward a common multi-service Internet protocol/multi-protocol label switching (IP/MPLS) network. Previously, such service providers maintained and operated separate legacy circuit-switched and packet-switched networks. Voice services were supported over the circuit-switched Public Switched Telephone Network (PSTN). Packet-switched networks, such as Frame Relay (FR) and Asynchronous Transfer Mode (ATM), were used in virtual private networks (VPN) for enterprises. These service providers are now migrating legacy Layer-2 and Layer-3 services to converged IP/MPLS-enabled IP networks.
Since the days of the telegraph there has been a need for a network system to provide sufficient quality to an end-user's application. Over time, two main techniques have evolved, Traffic Prioritization and Network Connection Admission Control (NCAC). Traffic Prioritization transmits some traffic ahead of other traffic. In the case of congestion in the network traffic marked with a higher-priority gets through, while lower priority traffic gets queued or dropped. This technique is quite effective, especially if the low-priority traffic is loss-insensitive, such as email which will be retransmitted at a later time if it cannot get through now. However, when the congestion is caused by traffic all in the same class then quality will degrade across all sessions or calls using that traffic class. When this traffic belongs to an application that is sensitive to loss, such as voice or video, the application cannot recover and user experience degrades across all sessions traversing the congested portion of the network. For these cases, an NCAC technique is used, perhaps in addition to a traffic prioritization technique.
Historically, in a circuit-switched network, such as the traditional voice telephony network, NCAC is inherent in the technology. An entire circuit is nailed up from end to end. Of course not all the bandwidth in that circuit is used at all times. In fact, for voice, very little is used. By allocating entire circuits, or time-slots in a TDM scheme such as SONET or SDH, quality can be preserved. But the cost of over allocating resources to handle the offered load is significant.
Ensuring a circuit-switched network recovers from failure requires reserving a backup path between each pair of nodes in the network, SONET's working and protect paths. This doubles the resources required to support the network's load, and is quite expensive.
With the evolution of networks, packet and cell-switched networks have evolved which allow the previously tied-up network resources to be used more efficiently through dividing the data up into bits or packets which can then be transmitted through the network. With these small atoms of data, previously unused resources can be used, radically increasing the efficiency of the network. In addition, doubling the bandwidth on every hop with a working and protect path in a ring is no longer required.
Intelligent interior gateway routing protocols (IGP's), such as OSPF and ISIS can be used to reroute traffic in the case of failure along a multitude of different paths. The downside of packet-switched networks is that their efficiency and robustness come without a clear mechanism to allocate resources required by the end applications, an NCAC system. Various NCAC systems have been proposed, however they all have significant shortcomings. Either they attempt to emulate the PSTN's circuit model, which comes with attendant problems of over provisioning, signaling load and complexity, or they attempt to pass all calls through a central device, which has problems in scaling, support and complexity. What is missing is an NCAC mechanism that does not succumb to the same problems that existing mechanisms do.
Network reliability and availability rank among the top concerns of most service providers. Maintaining revenue-generating service offerings is extremely important to them. For example, a one minute network outage that affects a hundred customers could cost a service provider several hundred thousand dollars. The high availability of packet-switched networks is a prerequisite to offering reliable and profitable carrier-class services.
A typical IP/MPLS-based network comprises routers and switches interconnected by fiber links and other transport facilities. Customers connect to the backbone (core) network through multi-service provider edge (PE) routers. Core routers in the backbone provide high-speed transport and connectivity between the PE routers. PE router line-cards and physical interfaces provide ATM, FR, Ethernet, IP/MPLS VPN's, and other Layer-2 and Layer-3 services. A switching fabric, at the heart of a router, is used to switch packets between the line cards.
IP routing protocols are used to advertise network topology, exchange routing information, and calculate forwarding paths between routers within and between network routing domains. IP routing protocols include Open Shortest Path First (OSPF), Intermediate System-to-Intermediate System (IS-IS), and Border Gateway Protocol (BGP).
IP/MPLS signaling protocols are used to establish, maintain, and release label-switched paths (LSP). MPLS-labels can be used in a “Switching” or “Outer” label context, where the router uses the label to determine the next-hop, or they can be used in a “Service” or “Inner” label context where the router uses the label to determine which VPN the packet belongs to. Including redundant network elements adds to the overall network cost. So service providers use different levels and types of fault tolerance in the edge and core network. Alternative paths can be quickly established around a failure point. Extra routers and links can be used to provide additional fault tolerance.
On the network's edge, thousands of customers may be connected through a single edge router. It represents a single point of failure, and is often the most vulnerable point of their network. Redundant control processor cards, line cards, and links, in each edge router can significantly improve fault tolerance.
Customer downtime can result from failures of access ports, edge links, edge routers, backbone transport facilities, or the core routers. Generally, the core network will provide a higher level of fault tolerance than will the edge network. The edge router is an important network element because it routes traffic to/from multiple customers to the core network. Improving the availability of edge routers is extremely important.
The main job of a network is satisfying customer expectations, e.g., availability of service according to service-level agreements (SLA). A network point-of-view deals with reducing network equipment and operation costs. The best networks satisfy service reliability and availability objectives while minimizing network equipment and operational costs.
The rush in the telecomm market's move towards IP/MPLS based networks left questions about service delivery and how to assure end-user quality of experience. Packet-switched networks do not provide the assurances inherent in its predecessor ATM or TDM networks. Many protocols and technologies are now being added to packet-switched networks to approximate some aspects of the ATM or TDM environment.
Admission controls allow services only if the network state can handle the required bandwidth without affecting existing traffic. Such is a simple idea, but very complex in its implementation, given the variability of packet-switched networks.
Admission control within the IP/MPLS core is materializing slowly but steadily. Topology learning and monitoring, and bandwidth reservation are essential. This is far more complicated than learning a static TDM network. IP networks by their nature are dynamic, so a dedicated listener is required to keep track of the changing topology. In VoIP and other applications, session based admission is not reasonable due to the many sessions required.
Admission controls are only needed to handle cases where the network congests. If a network never congests, it does not need an admission mechanism. But real-world networks are run much closer to their limits because of construction costs and/or usage growth. There is a need for methods and equipment to provide network connection admission control (NCAC) for end-to-end quality of service (QoS).